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Here, a space is called hereditarily locally connected ("hlc"), if each subset is locally connected.
Note that this is much stronger than the usual definition, which requires only connected subspaces to be locally connected. For instance, $\mathbb R$ fulfills the latter property, but is not hlc.

It is hard to find any papers dealing with hlc, since Google lists mainly examples for this much weaker property. Therefore, any references would be very welcome!

The only reference to hlc, I found, is this MO post, where it is also asked for references. Unfortunately, no answers.

My main question is:
Does there exist a non-discrete $T_2$ space, which is hlc?

Notes

  1. Call a space partly connected, if all components are open. It easy to see, that a space is locally connected, iff all open subsets are partly connected. Hence, a space is hlc, iff all subsets are partly connected. This is helpful to prove the following:

  2. There are many more examples of hlc spaces:
    Noetherian spaces, Alexandrov, filter like, hereditarily connected, arbitrary sums of hlc spaces, etc. But all these are discrete or not $T_2$.

  3. Obviously, in an hlc space, every totally disconnected subset is discrete. But as far as I could see this is not very helpful, but I didn't find a T2 space with this weaker property, either.

  4. In particular, an hlc $T_2$ space must not contain any non-trivial convergent sequence. Hence, if non-discrete, it cannot be sequential, first countable etc.

  5. If it is even regular $T_2$ and hlc, any countable subset is closed discrete. But I couldn't even prove that regular $T_2$ hlc spaces do not exist. As pointed out by Jakobian, an hlc $T_2$ space does not contain any infinite countably compact subspace.

  6. Since a locally connected, non-discrete space contains a non-discrete component, w.l.o.g we may assume that the space is connected.

  7. A (perhaps) slightly weaker condition than hlc is "each closed subset is locally connected". I'm also not aware of any non-discrete $T_2$ example. There is definitely no compact $T_2$ example. See also this MO post.

Ulli
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  • This does not answer your question, but just for information if one drops the Hausdorff condition. A space that is "hereditarily connected" (= every subset is connected) (see https://topology.pi-base.org/properties/P000196 for characterizations and theorems) is automatically hereditary locally connected. – PatrickR Jul 29 '25 at 20:50
  • @PatrickR: I added your example to note 3. Thank you! But these spaces are definitely not $T_2$ (not even $T_1$). – Ulli Jul 29 '25 at 21:26
  • Can you explain why regularity implies every countable subset is closed discrete? I cannot immediately see a proof. – David Gao Aug 02 '25 at 10:29
  • (In any case, there are $T_3$ non-discrete spaces in which all countable subsets are closed discrete, so that’s not really an obstruction.) – David Gao Aug 02 '25 at 10:40
  • @David Gao: If $A$ is countable, regular, then it is totally disconnected. Since every subset is locally connected, $A$ is discrete. Hence, $A \cup {x}$ is discrete for each $x \in \overline{A}$. Thus, $A$ is closed. – Ulli Aug 02 '25 at 14:15
  • @Ulli I see. Thanks! – David Gao Aug 02 '25 at 14:18
  • @Jakobian: That sounds interesting: could you please add a hint, that a non-discrete hlc T2 space needs to be uncountable. I think the non-pseudoradial argument I can follow, but how to proceed from there? – Ulli Aug 04 '25 at 21:23
  • @Jakobian: I see. But, if it's too complicated for a comment, would you mind posting it as a partial answer? Although I certainly won't accept it as an answer, but so far I found no argument to exclude the countable case. Anyway, I have to go to bed now, so I will not proceed on this today. – Ulli Aug 04 '25 at 21:43
  • Levy constructs some spaces which are not first countable at any point in Countable spaces without points of first countability, but they're all zero-dimensional – Jakobian Aug 07 '25 at 20:52
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    The spaces in Spaces in which the nondegenerate connected sets are the cofinite sets by Gruenhage seem to be related. They also posses a lot of properties like here. Of course such space wouldn't be locally connected. It suggests that we should perhaps assume CH or MA to answer this question. – Jakobian Aug 07 '25 at 21:35
  • Can anyone summarize what was wrong with the answer that was posted but then deleted? I read through it once but didn't immediately see any error. Presumably there was (at least) one though, since it was deleted. – pyridoxal_trigeminus Aug 08 '25 at 02:01
  • @R. H. Vellstra: because it was posted just before I went sleeping, I didn't check in detail. But as far as I recall, the user tried to prove that an arbitrary Hausdorff space contains an almost (non-discrete) subset, which is false. I think it failed in proving discreteness of the transfinite sequence. – Ulli Aug 08 '25 at 05:39
  • @Jakobian: I know Gruenhage's paper, but my impression is that these spaces are not of any help here. Even not for the weaker condition, that all totally disconnected subsets are discrete, since such a $T_2$ space cannot be biconnected. But I agree, CH might be relevant as for instance: (CH) there is an Efimov space (hence even compact $T_2$), such that all closed totally disconnected subsets are discrete (Fedorchuk). But, as pointed out above, it cannot be hlc. I would definitely be happy, to see just any consistent example! – Ulli Aug 08 '25 at 05:51
  • @R.H.Vellstra you need to be more specific since there is 3 such answers and all of them are wrong – Jakobian Aug 08 '25 at 12:06
  • @Jakobian Ah, I see. The one I remember reading attempted to use a neighborhood basis at a non-isolated point to construct a totally disconnected subset of a T2 hlc space that is not discrete, arriving at a contradiction. – pyridoxal_trigeminus Aug 08 '25 at 14:37
  • @R.H.Vellstra this is still not very specific because there are two deleted answers like that. One of them was wrong because it didn't guarantee that points wouldn't suddenly become non-isolated (like with cofinite topology when going from finite to infinite amount of points). The other one took one point from each neighbourhood but this one was definitely wrong since you can pick literally every point of the space this way. Alex Ravsky answer below indicates that they were close, if only they took open sets instead of points – Jakobian Aug 08 '25 at 16:47

1 Answers1

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My main question is:
Does there exist a non-discrete $T_2$ space, which is hlc?

I have a proof of the negative answer which looks too simple to be true. Indeed, suppose for a contradiction that $X$ is such a space and $x^*\in X$ be any nonisolated point. Let $x_0$ be any point of $X$ distinct from $x^*$. Since the space $X$ is Hausdorff, there exists an open neighborhood $U_0$ of $x_0$ such that $x^*\not\in\overline{U_0}$. Now we proceed by transfinite induction. At its $\alpha$-th step put $V_\alpha=X\setminus\overline{\bigcup_{\beta<\alpha} U_\beta}$. If $x^*\in V_\alpha$ then $V_\alpha$ is an open neighborhood of the point $x^*$. Since the point $x^*$ is nonisolated, there exists a point $x_\alpha\in V_\alpha$ distinct from $x^*$. Since the space $X$ is Hausdorff, there exists an open neighborhood $U_\alpha\subset V_\alpha$ of $x_\alpha$ such that $x^*\not\in\overline{U_\alpha}$.

When $x^*\not\in V_\alpha$ we are done. Put $Y=\{x^*\}\cup\bigcup_{\beta<\alpha} U_\beta$. The construction ensures that for each $\beta<\alpha$, the set $U_\beta$ is both open and closed subset of $Y$. For each neighborhood $U$ of $x^*$ in $Y$ there exists $\beta<\alpha$ such that the intersection $U\cap U_\beta$ is nonempty (and is proper, open and closed subset of $U$), so $U$ is not connected, a contradiction.

Ulli
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Alex Ravsky
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    This looks good to me. At the end, you don't need two sets intersecting $U$, since just one gives a proper non-empty clopen subset of $U$. – Jakobian Aug 08 '25 at 14:59
  • @Jakobian Thanks, I updated the answer. – Alex Ravsky Aug 08 '25 at 15:09
  • If I understand correctly, your proof shows that every non-discrete $T_2$ space contains an almost discrete subspace (= has exactly one non-isolated point). But this is false. I don't see where the flaw is right now. Unfortunately, today I have only very limited time to check, and tomorrow no time at all. It's really a pity, since, if correct, I really would like to award the bounty to it, which is possible only for some more hours. – Ulli Aug 08 '25 at 18:37
  • Is $U_\beta$ really closed in $Y$? Or can there be a $\gamma < \beta$ such that $y \in U_\gamma \cap \overline{U_\beta}$ ? – Ulli Aug 08 '25 at 18:53
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    @Ulli how does it show that? As for your question, $U_\beta$ is closed because for $\gamma < \beta$ we have $U_\gamma\cap U_\beta = \emptyset$ from our induction, and $x^\ast\notin \overline{U_\beta}$ – Jakobian Aug 08 '25 at 19:15
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    @Ulli If you pick $x_\beta\in U_\beta$, note that ${x^\ast}\cup {x_\beta : \beta < \alpha}$ doesn't need to be almost discrete, $x^\ast$ is only adjacent to whole of $\bigcup_{\beta < \alpha} U_\beta$, you can't take away those open sets and replace them with points. I believe that is what fixes this argument, and distinguishes it from other attempts at this question - it's almost like others argument but better – Jakobian Aug 08 '25 at 19:17
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    @Ulli Jacobian already addressed the first issue. Concerning the second, for each $\beta<\alpha$, $U_\beta$ is closed in $Y$, because $Y\setminus U_\beta={x^}\cup\bigcup_{\beta\ne\gamma<\alpha} {U_\gamma}$, but $x^\not\in \overline{U_\beta}$ by the construction of $U_\beta $. Also if $\gamma<\beta$ then the intersection $ U_\gamma \cap U_\beta$ is empty, and so is the intersection $ U_\gamma \cap\overline{ U_\beta}$, because $U_\gamma$ is open. – Alex Ravsky Aug 08 '25 at 19:22
  • Concerning the bounty. As far as I know, even if you reject to award my answer, it will be awarded by a half-size bounty automatically, because my answer is unique and has score at least $2$. – Alex Ravsky Aug 08 '25 at 19:22
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    @Jakobian: thank you for clarifying my completely false idea, that Y is almost discrete, and removing my doubt. Now, I'm almost convinced! But I will comprehensively check only on Sunday. @ Alex: just one minor issue: in the very last step, $U$ should be an open neighborhood in $Y$, isn't it? (Perhaps you already had this in mind?). – Ulli Aug 08 '25 at 19:31
  • @Ulli Right, $U$ is a neighborhood of $x^*$ in $Y$. I added this to the answer. – Alex Ravsky Aug 08 '25 at 19:34
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    @Alex: to be on the safe side, I awarded the bounty to your great answer. Please understand that I will only accept it after a detailed check on Sunday. – Ulli Aug 08 '25 at 19:35
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    @Ulli OK. I think this is a wise decision. My long mathematical experience shows that even simply and intuitive looking proofs can contain errors, hidden for decades. – Alex Ravsky Aug 08 '25 at 19:37
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    @Alex: Eventually I got some time to go through your proof, which really convinces me. As you said, it's surprising, how simple it is, once you get the right idea. So, thank you very much for this great proof. And apologies for my silly questions on Friday, where I was already tired and under time pressure. And yes, the bounty was very well deserved! – Ulli Aug 10 '25 at 18:46